A process for pulping a lignocellulosic material, for example, wood, straw or bagasse, by using an alkaline sulfide cooking liquor containing, as main components, sodium sulfide and sodium hydroxide at an elevated temperature, is referred to as an alkaline sulfide pulping process. This alkaline sulfide pulping process, which includes kraft process, is a most important chemical pulping process due to its advantage in that the quality of the resultant pulp is higher than that of another pulping processes, for example, a sulfite pulping process. However, in the other hand, the conventional alkaline sulfide pulping process has a disadvantage in that the yield of the resultant pulp is relatively small.
In order to eliminate the above-mentioned disadvantage of the conventional alkaline sulfide pulping process, various approaches were looked into for accelerating the delignification reaction between the lignocellulosic material and the pulping liquor and for preventing the decomposition of the carbohydrates in the lignocellulosic material. In one approach for this purpose, a polysulfide compound, sodium borohydride, hydrazine, amine compound, aldehyde compound or nitrobenzene compound were added to the alkaline sulfide pulping liquor. In another approach, the wood chips were pretreated with hydrogen sulfide. In a further approach, the so-called alkafide method was developed. However, all of the above-mentioned approaches, except for the polysulfide process, have not yet been practically utilized due to the fact that the approaches cause the pulping apparatus to be expensive or complicated, the cost of the pulping operation to be very high, or the processability of the pulping process to be poor, or result in an environmental pollution or exhibit a poor effect in pulping hardwood.
Recently, since B. Bach and G. Fiehn, Zellstoff und Papier, vol 21, No. 1, pages 3 to 7 (1972) and related East German Pat. No. 98,549 disclosed that the yield of pulp in the alkaline pulping process could be increased by adding an anthraquinone compound to the alkaline pulping liquor, various processes in which various anthraquinone compounds were used, were developed. For example, U.S. Pat. No. 3,888,727 disclosed a two-stage pulping process which comprised a first soda stage and second oxygen alkali stage or a first kraft stage and second oxygen-alkali stage, and in which sodium anthraquinone-2-sulfonate (AMS) was added to the treating liquor in the first stage. Canadian Pat. No. 986,662 disclosed a pulping process in which the lignocellulosic material was pre-treated with an alkali solution containing anthraquinone-2-monosulfonic acid. Japanese Patent Application Laying-open (KOKAI) No. 51-43403 disclosed a process in which a quinone compound was added to an alkali cooking liquor for a pulping process. West German Patent Application Laying-open (Offengungsschrift) No. 2,610,891 disclosed an oxygen-alkali pulping liquor containing a water-soluble oxygen carrier consisting of a quinone compound or hydroquinone compound. U.S. Pat. No. 4,012,280 disclosed an alkaline pulping liquor containing a sulphur free cyclic keto compound. U.S. Pat. No. 4,036,680 disclosed a soda pulping liquor containing a quinone compound and a nitro aromatic compound. Also, Japanese Patent Application Laying-open (KOKAI) No. 51-112903 disclosed a sulfite pulping process, wherein a cooking liquor contained a quinone compound.
In the above-mentioned prior arts, the quinone or hydroquinone compound alone or a combination of the quinone or hydroquinone compound and oxygen or an oxidizing agent were used for accelerating the delignification reaction and increasing the yield of the resultant pulp.
Furthermore, U.S. Pat. No. 4,036,680, issued to H. H. Holton, disclosed a soda pulping method in which a soda cooking liquor contains both a nitro aromatic compound and a diketohydroanthracene compound selected from unsubstituted and lower alkyl-substituted Diels-Alder adducts of naphthoquinone and benzoquinone. However, this method can not be applied to the pulping process in a reducing medium, such as the alkaline sulfide pulping process. This is because, when the nitro aromatic compound is added to the alkaline sulfide cooking liquor containing, as main components, sodium sulfide and sodium hydroxide, the nitro aromatic compound oxidizes the hydrosulfide ion derived from the sodium sulfide in the cooking liquor as reported by Svensk Papperstid, 71(23), 857-863(1968), so as to cause the sulfidity of the cooking liquor to be decreased. That is, the nitro aromatic compund itself is reduced so as to form a non-reactive compound.
It is already known from U.S. Pat. No. 2,938,913 that the diketohydroanthracene compound is readily oxidized by very mild oxidizing agents, for example, nitro compounds, hydrogen peroxide, chromic acid and air, so as to form an anthraquinone compound. Accordingly, it is evident that in the cooking liquor of the U.S. patent of Holton, the diketohydroantracene compound is oxidized into the anthraquinone compound by the nitro aromatic compound during the soda pulping process. That is, the process of the U.S. patent of Holton in which the combination of the nitro aromatic compound and the diketohydroanthracene compound is used, is substantially the same as the older soda pulping process in which the combination of the nitro aromatic compound and the anthraquinone compound is used. It is clear that the soda pulping process of the U.S. patent of Holton is carried out in an oxidizing condition.
The inventors of the present invention thoroughly studied the U.S. patent of Holton and found the fact that the addition of the combination of the nitro aromatic compound and the diketohydroanthracene compound to the alkaline sulfide pulping liquor which is in a reducing condition, caused the delignification reaction rate and the yield of the resultant pulp to be decreased, and the quality of the resultant pulp to become poor. That is, the combination of the nitro aromatic compound and the diketohydroanthracene compound is effective only for the soda pulping process which is carried out without using a reducing agent. The inventors also found the fact that, in the soda pulping process, the use of the diketohydroanthracene compound alone is not always more effective for increasing the delignification reaction rate and the yield of the resultant pulp than the use of the anthraquinone compound alone.
The inventors also studied in detail the pulping process using a cooking liquor containing a quinone compound. As a result of this study, it was found that Na.sub.2 S and NaHS in the cooking liquor is active as a reducing agent only when the cooking liquor is in a weak alkaline condition or neutral condition and can reduce the quinone compound into the corresponding hydroquinone compound. For example, in the pulping process as disclosed in Japanese Patent Application Laying-open (KOKAI) No. 51-112903, a lignocellulosic material is treated with a sulfite cooking liquor containing a quinone compound at an elevated temperature under a pressurized condition. In this case, before the delignification reaction on the lignocellulosic material has occurred, a deacetylation reaction or peeling reaction of the lignocellulosic material occurs. This deacetylation or peeling reaction causes the alkali in the alkaline sulfite cooking liquor to be consumed. As a result of this consumption, the sulfite cooking liquor exhibits a weak alkaline or neutral condition. Under this condition, NaHS can exhibit a high reducing activity and accelerate the reduction of the quinone compound into the corresponding hydroquinone compound.
However, it was also found by the inventors that in a strong alkaline cooking liquor, Na.sub.2 S and NaHS can not exhibit the reducing activity. For example, in the alkaline sulfide pulping process, the cooking liquor containing sodium sulfide (Na.sub.2 S) and sodium hydrogen sulfide (NaHS) and sodium hydroxide can maintain its strong alkaline condition constant over the entire period of the delignification reaction. Accordingly, in the alkaline sulfide pulping process, the sodium sulfide can not exhibit the reducing activity for the quinone compound. Generally, the quinone compound such as naphthoquinone and anthraquinone has a very small solubility in the alkaline sulfide cooking liquor and only a small amount of the quinone compound can be reduced by carbohydrates in the liqnocellulosic material into the corresponding hydroquinone compound which is generally soluble in the alkaline sulfide cooking liquor. The above-mentioned pulping process causes the lignin compounds in the lignocellulosic material to be converted into lignin radicals. The small amount of the resultant hydroquinone compound scavenges the lignin radicals so as to accelerate the delignification of the lignosellulosic material and the hydroquinone compound per se is oxidized into the quinone compound. That is, in the conventional delignification mixture, a redox oxidation-reduction system of the quinone compound and the corresponding hydroquinone compound is formed in the presence of the carbohydrates. However, this redox system is very small and, therefore, can not significantly accelerate the delignification of the lignocellulosic material.
As stated above, since the quinone compound can merely be reduced in a very small amount in the alkaline sulfide cooking liquor, it is clear that the quinone compound is not highly effective for accelerating the delignification of the lignocellulosic material with the alkaline sulfide cooking liquor.
It is also known that, in the conventional alkaline sulfide pulping process, an inorganic reducing compound, such as sodium sulfite, is not only ineffective for promoting the delignification but also tends to retard the delignification of the lignocellulosic material and to decrease the yield of the resultant pulp.